1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, e.g., a semiconductor laser printer.
2. Description of the Related Art
Japanese Patent Laid-open Publication No. 05-6112 discusses an image forming apparatus in which, when an image is not formed, a reference voltage to be applied to a transfer member when an image is formed is set (hereinafter referred to as automatic transferring voltage control (ATVC)). More specifically, in the ATVC, a present transfer current value is detected to set a reference transferring voltage value at which the current value becomes a target value. A plural repetition of the above operation sets a suitable transfer output voltage when an image is formed.
Now, control sequence of the ATVC is described below with reference to FIG. 9. After a scanner motor is started to be driven, the semiconductor laser is continuously or intermittently forced to be lighted up (hereinafter referred to as forcible light emission) to detect a rotation speed of the scanner motor by a light-receiving sensor provided over a laser scanning line. According to the above detection, when a polygon mirror comes to be constantly rotated, the semiconductor laser repeats a light up and a light out alternately according to a synchronization signal input into the light-receiving sensor (hereinafter referred to as unblanking luminescence) so as not to cause the semiconductor laser to irradiate onto a surface of a photosensitive member (i.e., image area). Subsequently, after the processing is shifted to the unblanking luminescence, the ATVC is executed.
When a toner image is actually transferred, a case where an exposed portion of the photosensitive drum does not come into contact with (reach) a transfer nip portion is determined as a reference state and the transferring voltage value is set so as to be adjustable based on the reference transferring voltage value set with respect to the reference state. For example, in a case where a recording medium that receives a toner image heads into the next transfer nip portion, the transferring voltage value after the recording medium heads into the transfer nip portion is set with reference to the transferring voltage value (i.e., reference transferring voltage) in a state before the recording medium heads into the transfer nip portion. For the reason described above, the image forming apparatus normally performs the ATVC after shifting a control to the above described unblanking luminescence.
In the recent image forming apparatus, reduction of a first print out time before a first page of a print product is discharged after a print instruction is received is increasingly demanded. In association therewith, an early completion of the above described ATVC is also demanded. For example, in the ATVC control illustrated in FIG. 9, if the ATVC is started at the same time the printing is started, the above demand can be realized.
In a charged surface of the photosensitive member, potentials change between a case of a surface of the photosensitive member on which the semiconductor laser is forcibly emitted and a case of a surface of the photosensitive member subjected to an unblanking luminescence and thus current values flowing through a transfer member contacting the photosensitive member also change. More specifically, if the ATVC is performed while the semiconductor laser is forcibly emitted, the above described potential change (i.e., change of the transfer current value flowing through the transfer member) occurs, which reduces accuracy of the ATVC. Alternatively, a set value itself obtained in the ATVC becomes an improper value.
On the other hand, a method in which the current value is set with respect to the target voltage value is also known as a transferring method in the image forming apparatus in addition to a method in which the voltage value is set with respect to the target current value. The above described problems also occur in this case.